Method for controlling supply of fuel to a combustion engine

ABSTRACT

A method for controlling supply of fuel to a combustion engine, e.g. a self-igniting internal combustion engine in a vehicle, having a first group of cylinders and a second group of cylinders, the method comprising the steps of: determining if a demanded total fuel quantity to the combustion engine is below a first predetermined total fuel quantity; and, if the demanded total fuel quantity to the combustion engine is below the first predetermined total fuel quantity, increasing the fuel supply to the first group of cylinders with a value determined by the demanded total fuel quantity and decreasing the fuel supply to the second group of cylinders with substantially the same value. Also, a second method, a computer program and an electronic control unit.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a 35 U.S.C. §§ 371 national phase conversionof PCT/SE2003/001810 filed 24 Nov. 2003, which claims priority ofSwedish Application No. 0203476-7 filed 26 Nov. 2002. The PCTInternational Application was published in the English language.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods for controlling fuel supply toa combustion engine. The invention also relates to a computer program,an ECU (Electronic Control Unit) and a computer program product forperforming the methods.

BACKGROUND OF THE INVENTION

Electronic control of fuel injection to combustion engines in vehiclesis used today due to the advantages enabled through the electroniccontrol in comparison with a mechanical control system. Electroniccontrol has significantly contributed to make e.g. the diesel enginemore powerful, more efficient, cleaner and quieter. U.S. Pat. No.5,131,371-A discloses a part of such an electronic control system for adiesel engine.

A demanded fuel supply to a diesel engine is typically substantiallyproportional to a requested engine torque. The actual fuel supplied tothe cylinders of the engine shall ideally be directly proportional tothe demanded fuel supply. Hence the actual fuel supply shall ideally beproportional to the requested engine torque. However, in some fuelinjection systems the correspondence between the actual fuel supply andthe demanded fuel supply has not been satisfying during certain workconditions and demanded fuel quantities. This unsatisfyingcorrespondence is caused by hydraulic instability in the fuel injectionsystem, where the hydraulic instability may be caused e.g. when a fuelquantity control valve in the fuel injection system closes a fuelpassage in the valve. The valve may for instance comprise a closingmeans, which is forced against a stop surface when closing the passage.The passage needs to be closed rapidly and the closing means maytherefore bounce on the stop surface when closing the passage, thusenabling undesired leakage of fuel through the passage. This leakagecauses the actual fuel quantity to differ from the demanded fuelquantity. If the demanded fuel quantity is small, the leakage isrelatively high compared to the demanded fuel quantity. This makes thefuel injection control more difficult. A driver of the vehicleexperiences the hydraulic instability through e.g. undesirable anddistracting noise.

In some systems, hydraulic instability causes problems when a smallincrease of fuel supply is demanded and the current fuel supply isrelatively low. The hydraulic instability here causes a decrease oftorque and actual fuel supply although a higher fuel supply than in theinjection cycle before is demanded (see also FIG. 1). To avoid thedanger of an unstable fuel injection control system caused by thisnon-linear correspondence between the demanded fuel supply and theactual fuel supply, the fuel injection control system must be morestability robust than it would have to be if the non-linearity would notexist. There also has to be higher demands on the insensitivity of thefuel injection control system in order to keep it sufficiently accurateand reject disturbances. A way to compensate for the non-linearity is todevelop a compensation routine for the control system, but this adds tothe complexity and the computing time and is not accurate since therange of non-linearity depends on the individual vehicle configurationand the temperature of the fuel.

In order to avoid the hydraulic instability problems during the mostfrequently used driving conditions, fuel injection systems usually aredesigned in such a way that the hydraulic instability affects the fuelinjection system within a range of low engine torque values. The fuelsupply in this range is preferably designed to be lower than the fuelsupply during idle speed. Hydraulic instability is however likely toaffect the fuel supply also in ranges above idle speed. There aredriving conditions wherein the fuel supply may be within the rangeswhere instability occurs, such as during cruise control at relativelylow engine torque and during electronically controlled automatic orsemi-automatic gear shifting in a smooth way.

SUMMARY OF THE INVENTION

An object of the present invention is to decrease vibration and noisecaused by a combustion engine during certain driving conditions in e.g.a vehicle, such as during cruise control at relatively low engine torqueand when automatically or semi-automatically shifting gear through agearbox connected to the engine.

Another object of the invention is to enable a stability robust andinsensitive control system also in fuel quantity ranges where hydraulicinstability occurs.

Yet another object of the invention is to enable smoother driving of anengine during certain driving conditions.

The invention relates to a method for controlling supply of fuel to acombustion engine, e.g. a self-igniting internal combustion engine in avehicle, having a first group of cylinders and a second group ofcylinders. The method comprises according to a first aspect the stepsof:

determining if a demanded total fuel quantity to the combustion engineis below a first predetermined total fuel quantity;

and, if the demanded total fuel quantity to the combustion engine isbelow the first predetermined total fuel quantity, increasing the fuelsupply to the first group of cylinders with a value determined by thedemanded total fuel quantity and decreasing the fuel supply to thesecond group of cylinders with substantially the same value. The valueshall here of course be understood as an absolute value. Through themethod it is achieved that a fuel quantity range or ranges below thefirst predetermined total fuel quantity and in which range/rangeshydraulic instability occurs may be avoided by letting the fuel quantityinjected into the first group of cylinders be above the range and thefuel quantity injected into the second group of cylinders be below therange, without affecting the average fuel quantity injected into thecylinders. To group all or some of the cylinders of the engine into thefirst and the second group shall be understood as any predeterminedgrouping of the cylinders, regardless of the basis for the grouping. Thecylinders can belong to one of the first and second group due to e.g.physical position related to each other; a common fuel quantityactuator; other hydraulic, pneumatic or electric control means incommon; and due to any other predetermine constructional or abstract“rule” implemented for the control of the engine, such as ignitionorder, where e.g. every second cylinder in the ignition order belongs tothe first group and the remaining cylinders belong to the second group.

The value may be reciprocally proportional to the demanded total fuelquantity on at least a part of a demanded total fuel quantity rangebetween zero demanded total fuel quantity and the first predeterminedtotal fuel quantity. Hereby is achieved that that the increase anddecrease respectively injected into the cylinders for at least a part ofthe total fuel quantity range increase as the demanded total fuelquantity becomes lower.

The value may be reciprocally proportional to the demanded total fuelquantity in the whole demanded total fuel quantity range between asecond predetermined total fuel quantity and the first predeterminedtotal fuel quantity. Hereby is achieved that the relative increase of anoffset from a mean fuel quantity for the cylinder increases when thedemanded total fuel quantity decreases.

The value may also be highest and constant in a demanded total fuelquantity range between a second predetermined total fuel quantity and athird predetermined total fuel quantity, which is larger than the secondpredetermined total fuel quantity, but smaller than the firstpredetermined total fuel quantity.

The method may according to a second aspect comprise the steps of:

determining if a demanded fuel quantity to a cylinder is below a firstpredetermined fuel quantity;

and, if the demanded fuel quantity to the cylinder is below the firstpredetermined fuel quantity, increasing the fuel supply to the firstgroup of cylinders with a value determined by the demanded fuel quantityand decreasing the fuel supply to the second group of cylinders withsubstantially the same value.

The value may also here be reciprocally proportional to the demandedfuel quantity on at least a part of a demanded fuel quantity rangebetween zero demanded fuel quantity and the first predetermined fuelquantity.

The value may here be reciprocally proportional to the demanded fuelquantity in the whole demanded fuel quantity range between a secondpredetermined fuel quantity and the first predetermined fuel quantity,the second predetermined fuel quantity being smaller than the firstpredetermined fuel quantity.

The value may be highest and constant in a demanded fuel quantity rangebetween a second predetermined fuel quantity and a third predeterminedfuel quantity, which is larger than the second predetermined fuelquantity, but smaller than the first predetermined fuel quantity. Thesecond predetermined fuel quantity may be the zero demanded fuelquantity.

The fuel supply may be increased to every two cylinders of all cylindersof the engine and decreased to the other cylinders of the engineaccording to an ignition order for all the cylinders of the engine.Hereby is achieved that a relatively even torque is provided by theengine compared to an embodiment where the decrease and increase of thefuel supply to the respective cylinders is distributed in another way.

The value may for some embodiments always be less than 100%. Hereby isachieved that the fuel supply to the cylinders of the second group isnot shut off completely.

The steps of the method may be performed during at least a part of agear shifting procedure controlled by an electronic control unit forsemi-automatic or automatic gear shifting.

The steps of the method may alternatively or in addition be performedwhen an automatic cruise control system for a vehicle controls thecombustion engine.

The method also relates to a computer program comprising computerreadable code means, which when run on a computer for controlling fuelsupply to a combustion engine cause the computer to perform the steps ofthe first or second aspect of the method.

Furthermore, the invention relates to an ECU in a vehicle forcontrolling fuel supply to a combustion engine in the vehicle. The ECUcomprises a storing means and the computer program recorded thereon.

Moreover the invention relates to a computer program product, comprisinga computer readable medium, which comprises the computer program. Thecomputer program product may be a floppy disc, a DVD, a CD, a hard diskor any other non-volatile memory.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages and effects as well as features of the presentinvention will be more readily understood from the following detaileddescription when read together with the accompanying drawings, in which:

FIG. 1 is a diagram showing the actual fuel supply to one cylinder as afunction of a demanded fuel supply to that cylinder,

FIG. 2 is a schematic block diagram of a system according to oneembodiment of the invention,

FIG. 3 is an outline diagram of a fuel injection system that can be usedtogether with the invention,

FIG. 4 schematically shows an ECU for controlling the engine accordingto the invention,

FIG. 5 is a schematic flow diagram for a method according to theinvention, and

FIG. 6 is a diagram showing offset profiles according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

While the invention covers various modifications and alternativeconstructions, some embodiments of the invention are shown in thedrawings and will hereinafter be described in detail. However it is tobe understood that the specific description and drawings are notintended to limit the invention to the specific forms disclosed. On thecontrary, it is intended that the scope of the claimed inventionincludes all modifications and alternative constructions thereof fallingwithin the spirit and scope of the invention as expressed in theappended claims to the full range of their equivalents.

FIG. 1 shows an actual fuel quantity injected into a cylinder of aninternal combustion engine in a vehicle as a function of the demandedfuel quantity for that cylinder. The figure could for a diesel engine aswell show an actual engine torque contribution from the cylinder as afunction of a demanded torque contribution, since injected fuel quantityis closely related to the engine torque. It is to be understood that theterm ‘fuel quantity’ in the rest of the specification, including theclaims, could be replaced by ‘engine torque’ when applied to a dieselengine, since the spirit of the invention is the same regardless ofwhether ‘fuel quantity’ or engine torque’ is referred to regardingdiesel engines. Line L1 shows the ideal, linear correspondence betweenthe demanded fuel quantity and the actual fuel quantity. Line L2 is anexample of a real correspondence between the demanded fuel quantity andthe actual fuel quantity at relatively low fuel quantities. Due tohydraulic instability, an increased demand of fuel within a fuelquantity range A gives a decreased actual quantity of fuel. Thus it isundesirable to let the cylinders of the engine work in this range forreasons mentioned above.

FIG. 2 schematically shows a combustion engine 1 in the form of aninternal diesel engine in a vehicle 2, such as a truck and a bus,equipped with an even number of cylinders. The invention may be used inany suitable fuel injection system, such as an UPS (Unit Pump System), aCRS (Common Rail System) and an UIS (Unit Injector system). An ECU 3with an engine control computer program controls the fuel quantityinjected into each cylinder and may e.g. in the case of an UIS beelectronically connected to valves positioned in e.g. each unit injectoror at another position upstream the unit injectors for control of thefuel injection.

FIG. 3 is an outline diagram of the hydraulic part of an exemplary UISin which the invention can be utilised. In this example the engine 1comprises six cylinders 4 a-4 b with an associated unit injector each.Fuel is taken from a fuel tank 5 by a feed pump 6, which forces the fuelthrough a fuel filter 7 and a stop-valve 8. As can be seen in FIG. 3,the unit injectors and thereby also the cylinders are hydraulicallydivided into a first group and a second group with three cylinders/fuelinjectors each, 4 a and 4 b respectively. The fuel quantity supply andthe fuel injection time for the cylinders 4 a in the first group iscontrolled by the ECU 3 through two actuators 9 a-9 b downstream of thestop-valve 8, where one of the actuators, 9 b, is used for controllingthe fuel injection time and the other, 9 a, is used for controllinginjected fuel quantity. In the same way, two other actuators 9 c-9 dpositioned downstream of the stop valve 8 and hydraulically parallelwith the first two actuators 9 a-9 b are used by the ECU 3 to controlthe fuel injection time and the injected fuel quantity to the cylinders4 b of the second group. The actuators are for instance magnetic valves.

FIG. 4 schematically shows the ECU 3, which comprises a microcontroller10, which in this embodiment comprises a CPU (Central Processing Unit)and RAM (Random Access Memory) and at least one non-volatile memory 13,such as a ROM (Read-only Memory), an EPROM (Erasable Programmable ReadOnly memory) and a Flash memory. An engine control computer program 14is stored in the non-volatile memory and causes the ECU 3 to inter aliacontrol the fuel injection to the engine 1. Other software may as wellbe stored in the non-volatile memory 13, e.g., a cruise control computerprogram 15 and a vehicle speed limiting computer program 16. Themicrocontroller 10 is connected to a CAN (Controller Area Network)interface 17 via a first databus 18 a for communication with other ECUs,such as ECUs for an automatic gearbox system 19 (see FIG. 2), brakesystem and adaptive cruise control system, via a vehicle internal CAN 20(see FIG. 2). The microcontroller 10 is also connected to input signalcircuitry 21 via a second data bus 18 b for receiving signal fromdifferent ECU internal and external sensors (not shown) connecteddirectly to the ECU 3 and output signal circuitry 22 via a third databus 18 c for operating and sending signals to the actuators 9 a-9 d aswell as other actuators and relays. Furthermore, several types of datastoring means/computer program products 23 may be connected to themicrocontroller 10 through a fourth data bus 18 d. Such a storing means23 may be an EEPROM (Electrically Erasable Programmable Read OnlyMemory), a second ROM or a hard disk.

Having described an embodiment of a system in which a method accordingto the invention may be implementet, a method according to the inventionwill now be described in conjunction with FIG. 1, FIG. 5 and FIG. 6. Itmust be understood that the method described here can be incorporatedand used in combination with known computer program modules that may becomprised in the engine control computer program 14. Examples of suchcomputer program modules are an injected-fuel quantity limiting module,an idle speed control module, engine start control module, anintermediate-speed control module and an injected-fuel quantitycompensation module. In a first step S1 of a first aspect of a method,an instantaneous, demanded total fuel quantity is calculated by the ECU3 when the engine 1 is running. The demanded total fuel quantity isaffected by signals from sensors such as an accelerator pedal sensor andan engine speed sensor and signals from other vehicle systems such as abraking system, a stability system, and a traction control system. Themanual engine power demand indicated by the accelerator pedal sensor mayalso be overridden by a cruise control system, a vehicle speed limitingsystem or an automatic or semi-automatic gear shifting system. Thecalculation itself depends upon maps, which also take other influencesinto account, such as fuel and intake-air temperature. The maps and thecalculation are as such known in the art and are therefore not describedmore in detail. According to a second aspect of the method, anindividual demanded fuel quantity for each cylinder is calculated inaddition or alternatively to the demanded total fuel quantity. Thedemanded fuel quantity for a cylinder may e.g. be calculated by dividingthe demanded total fuel quantity with the number of cylinders.

Instead of the ECU 3, other ECUs such as the ECU 19 for an automatic orsemi-automatic gear shifting system may perform the calculation in stepS1 and send the output to the ECU 3. In other words, the method is notdependent upon where the calculation is performed as long as the unitthat performs the calculation is connected to the ECU 3.

After step S1, the method continues with a second step S2, in which,according to the second aspect, it is determined if the demanded fuelquantity for a cylinder, is below a value equal to a first predeterminedfuel quantity P. According to the first aspect, it is determined if thedemanded total fuel quantity to all the cylinders 4 a-4 b is below avalue equal to the first predetermined fuel quantity P times the numberof cylinders. The first predetermined fuel quantity P may be below therequired fuel quantity for idle speed I (see FIG. 1) but may also be setto any value above the idle speed dependent on e.g. the type of utilisedfuel injection system and driving condition in which the hydraulicinstability is likely to occur. If the demanded fuel quantity is abovethe first predetermined fuel quantity P and the demanded total fuelquantity is above the first predetermined fuel quantity times the numberof cylinders respectively, no adjustment of the demanded fuel quantityto each cylinder 4 a-4 b is considered necessary and the method returnsto step S1. If the demanded fuel quantity is below the firstpredetermined fuel quantity P and the demanded total fuel quantity isabove the first predetermined fuel quantity times the number ofcylinders respectively, the method continues with a third step S3.

In step S3, a value of an increase or decrease of the fuel quantitydistributed to each cylinder 4 a-4 b is determined by the ECU 3. Thereis no change of the demanded total fuel quantity calculated in step S1,but the substantially equal quantity distributed to each cylinder 4 a-4b is going to be changed into an unequal distribution between thecylinders 4 a-4 b. In the system described above, an increase of thefuel supply to the first group of cylinders 4 a is determined accordingto a curve/map stored in the ECU 3 and described below in conjunctionwith FIG. 6. In order to substantially preserve the demanded total fuelquantity to the engine 1, a decrease of the fuel supply to the secondgroup 4 b is also determined, where the decrease is substantially equalto the fuel supply increase for the first group.

In a fourth step S4, subsequent to step S3, the fuel supply to eachcylinder 4 a-4 b is calculated using the increase and decreasedetermined in step S3. In the case of a system according to FIG. 3, itis only necessary to calculate the fuel supply to each group and therebysimplify the calculation and indirectly calculate the fuel supply toeach cylinder 4 a-4 b as each cylinder in a specific group is suppliedwith a substantially equal amount of fuel for a stroke cycle for whichthe calculation has been performed.

In step S5 after step S4, the ECU 3 controls the actuators in order tosupply the fuel, which was calculated in step S4, to each cylinder.After step S5, the method returns to step S1.

FIG. 6 discloses a fuel supply offset for a cylinder as a function ofthe demanded fuel supply for that cylinder as it was calculated in stepS1 in the second aspect of the method. The offset means a decrease ofthe fuel supply to the cylinder if the cylinders belong to the secondgroup and an increase if the cylinder belongs to the first group. 100%offset means a 100% decrease or 100% increase of fuel supply to thecylinder and thereby no fuel supply at all to the cylinder if thecylinder belongs to the second group or a doubled fuel supply to thecylinder if the cylinder belongs to the first group. Correspondingly 0%offset means no decrease or increase of the fuel supply to the cylinder.A first curve C1 shows a correspondence according to a first embodimentand comprises a straight horizontal part at the level of a 100% offsetfor a demanded fuel supply calculated in step S1 and being between asecond predetermined fuel quantity Z, which may be zero and a thirdpredetermined fuel quantity Q, which is smaller than the firstpredetermined fuel quantity P, but larger than the second predeterminedfuel quantity Z. In the range between Q and P, there is a reciprocallyproportional correspondence between the demanded total fuel supply andthe offset. This is shown as a straight, inclined second part of thefirst curve C1, where the offset is 100% at the third predetermined fuelquantity Q and 0% at the first predetermined fuel quantity P.

An inclined, straight, second curve C2 shows a second embodiment inwhich the offset is reciprocally proportional to the demanded fuelquantity calculated in S1 in the fuel quantity range between the secondpredetermined fuel quantity Z and the first predetermined fuel quantityP. The offset is 100% at the second predetermined fuel quantity Z andzero when the demanded fuel quantity is equal to the first predeterminedfuel quantity P.

A third curve C3, represent a third embodiment similar to the firstcurve C1, but here a straight horizontal part of C3 between the secondpredetermined fuel quantity Z and the third predetermined fuel quantityQ shows a smaller offset R, i.e. below 100%. In the range between Q andP, there is a reciprocally proportional correspondence between thedemanded total fuel supply and the offset. This is shown as a straight,inclined second part of the third curve C3, where the offset is R % atthe third predetermined fuel quantity Q and 0% at the firstpredetermined fuel quantity P.

An inclined, straight, fourth curve C4 shows a fourth embodiment inwhich the offset is reciprocally proportional to the demanded fuelquantity calculated in S1 in the fuel quantity range between the secondpredetermined fuel quantity Z and the first predetermined fuel quantityP. The offset is R % at the second predetermined fuel quantity Z andzero when the demanded fuel quantity is equal to the first predeterminedfuel quantity P. An offset calculation according to the third and fourthcurves, C3 and C4 respectively, is especially advantageous if a cruisecontrol system has taken over the control of the demanded fuel quantitysince a total shut-off of fuel injection to cylinders is undesiredduring cruise control. This is due to that the noise caused by hydraulicinstability is lower if the fuel supply is not completely shut-off to acylinder and because cruise control may go on for a relatively longtime.

Other types of curves may of course also be used, such as a non-linear,fifth curve C5.

It is obvious that instead of showing the offset for a cylinder as afunction of a demanded fuel quantity of a single cylinder in FIG. 6, theoffset as a function of the demanded total fuel quantity calculated instep S1 could have been appended instead of FIG. 6, since such analternative figure would show curves of the same types as the curvesC1-C5.

A demanded fuel quantity for a cylinder, in FIG. 1 shown as a fuelquantity E, within the range A may due to the hydraulic instabilitycreate problems for the control of the fuel injection. Through theoffset determination and calculation of a new demanded fuel quantity foreach cylinder according to the steps S3 and S4, the cylinders 4 a in thefirst group gets a higher quantity of fuel, which is above the range A.This is illustrated with a first dot D1. The cylinders in the secondgroup gets a lower quantity of fuel, which is below the range A. This isillustrated with a second dot D2. Hence problems associated with range Ais avoided.

In a UIS having a fuel quantity actuator for every cylinder of theengine, each cylinder can be controlled individually and not in groupsof three as in the system discussed above in conjunction with FIG. 3.However in both these types of UIS, every two cylinders in apredetermined ignition order for the cylinders may belong to the firstgroup and the remaining cylinders belong to the second group. It is ofcourse also possible to leave out one or more cylinders, so that theleft out cylinders are not affected by the invention. Such embodimentswould however not be as beneficial as if all the cylinders were affectedby the invention.

1. A computer program embodied in a computer-readable medium comprisingcode causing an apparatus to function to control a supply of fuel to acombustion engine having a first group of cylinders and a second groupof cylinders, wherein the code causes the apparatus to function to:determine when a demanded total fuel quantity to the combustion engineis below a first predetermined total fuel quantity; calculate a valuefor increasing and decreasing the fuel supply to the combustion engineas function of the demanded total fuel quantity; and increase the fuelsupply to the first group of cylinders by the value, and decrease thefuel supply to the second group of cylinders by substantially the samevalue, wherein the apparatus increases the fuel supply to the firstgroup of cylinders and decreases the fuel supply to the second group ofcylinders when the apparatus determines that the demanded total fuelquantity to the combustion engine is below the first predetermined totalfuel quantity.
 2. In combination, the computer program embodied in thecomputer-readable medium of claim 1, and a vehicle, including thecombustion engine; an electronic control unit in the vehicle, whereinthe electronic control unit includes the computer-readable medium thatembodies the computer program. and further wherein the electroniccontrol unit controls the fuel supply to the combustion engine in thevehicle.
 3. A method for controlling supply of fuel to a combustionengine having a first group of cylinders and a second group ofcylinders, the method comprising the steps of: determining if a demandedfuel quantity to one of the cylinders is below a first predeterminedfuel quantity; and, if the demanded fuel quantity to the one cylinder isbelow the first predetermined fuel quantity, increasing the fuel supplyto the first group of cylinders with a value determined by the demandedfuel quantity and decreasing the fuel supply to the second group ofcylinders with substantially the same value.
 4. A method according toclaim 3, wherein the value is reciprocally proportional to the demandedfuel quantity on at least a part of a demanded fuel quantity rangebetween zero demanded fuel quantity and the first predetermined fuelquantity.
 5. A method according to claim 4, wherein the value isreciprocally proportional to the demanded fuel quantity in the wholedemanded fuel quantity range between a second predetermined fuelquantity and the first predetermined fuel quantity, the secondpredetermined fuel quantity being smaller than the first predeterminedfuel quantity.
 6. A method according to claim 4, wherein the value ishighest and constant in a demanded fuel quantity range between a secondpredetermined fuel quantity and a third predetermined fuel quantity,which is larger than the second predetermined fuel quantity, but lowerthan the first predetermined fuel quantity.
 7. A method for controllingsupply of fuel to a combustion engine having a first group of cylindersand a second group of cylinders, the method comprising the steps of:determining if a demanded total fuel quantity to the combustion engineis below a first predetermined total fuel quantity; and, if the demandedtotal fuel quantity to the combustion engine is below the firstpredetermined total fuel quantity, increasing the fuel supply to thefirst group of cylinders with a value determined by the demanded totalfuel quantity and decreasing the fuel supply to the second group ofcylinders with substantially the same value.
 8. A method according toclaim 7, wherein the value is reciprocally proportional to the demandedtotal fuel quantity on at least a part of a demanded total fuel quantityrange between zero demanded total fuel quantity and the firstpredetermined total fuel quantity.
 9. A method according to claim 8,wherein the value is reciprocally proportional to the demanded totalfuel quantity in the whole demanded total fuel quantity range between asecond predetermined total fuel quantity and the first predeterminedtotal fuel quantity, the second predetermined total fuel quantity beingsmaller than the first predetermined total fuel quantity.
 10. A methodaccording to claim 8, wherein the value is highest and constant in ademanded total fuel quantity range between a second predetermined totalfuel quantity and a third predetermined total fuel quantity, which islarger than the second predetermined total fuel quantity, but lower thanthe first predetermined total fuel quantity.
 11. A method according toclaim 7, wherein the value is always less than 100%.
 12. A methodaccording to claim 7, wherein the steps are performed during at least apart of a gear shifting procedure controlled by an electronic controlunit for semi-automatic or automatic gear shifting.
 13. A methodaccording to claim 7, wherein the steps are performed when an automaticcruise control system for a vehicle controls the combustion engine. 14.A method according to claim 7, wherein the fuel supply is increased toevery two cylinders of all cylinders of the engine and decreased to theother cylinders of the engine according to an ignition order for all thecylinders of the engine.